Other Sources of One-Carbon Substituted Folates

Formylglutamate can transfer its formyl group directly onto tetrahydrofolate to yield 5-formyl-tetrahydrofolate. Formyl-glutamate is not a normal physiological intermediate, and the formation of 5-formyl-tetrahydrofolate is probably a side reaction of FIGLU formimino transferase. 

Free formate can react with tetrahydrofolate to form 10-formyl-tetrahy-drofolate the plasma concentration of formate rises in folate deficiency, and the ability to metabolize [ Cjformate has been used as an index of folate depletion in experimental animals. 

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Serine Hydroxymethyltransferase Serinehydroxymethyltrans-ferase is a pyridoxed phosphate-dependent aldolase that catalyzes the cleavage of serine to glycine and methylene-tetrahydrofolate (as shown in Figure 10.5). Serine is the major source of one-carbon substituted folates for biosynthetic reactions. At times of increeised cell proliferation, the activities of serine hydroxymethyltransferase emd the enzymes of the serine biosynthetic pathway cue increased. The other product of the reaction, glycine, is also required in increased cimounts under these conditions (for de novo synthesis of purines). 

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